1. Field of the Invention
The present invention relates to a control apparatus for an internal combustion engine which enables a motor vehicle to travel even with mixed fuel having an arbitrary ratio of alcohol (single composition) and gasoline, and in particular, to a technique of estimating a single composition concentration based on an air-fuel ratio correction amount obtained by using an output of an air-fuel ratio sensor in exhaust gas and optimizing a fuel injection amount by using a concentration estimation value.
2. Description of the Related Art
There is a motor vehicle called a flexible fuel vehicle (FFV) which is capable of traveling with mixed fuel of various compositions of alcohol and gasoline, in addition to with gasoline alone.
Alcohol has a C (carbon) atom content different from that of ordinary gasoline, and hence, when mixed fuel of alcohol and gasoline is supplied to an internal combustion engine used for the FFV, it is necessary to adjust the amount of fuel to be injected according to the alcohol concentration in fuel.
As such an FFV, there has hitherto been known an FFV in which estimation of the alcohol concentration in fuel is performed by using a correlation between the alcohol concentration and an air-fuel ratio feedback correction coefficient that is calculated based on an air-fuel ratio in exhaust gas detected by an air-fuel ratio sensor (see, for example, JP 2004-278449 A and U.S. Pat. No. 7,159,623).
There occurs a delivery delay before fuel in a fuel tank which has the alcohol concentration changed due to refueling arrives at an injector that supplies the fuel to an internal combustion engine, and the delay becomes larger in a returnless fuel supply system.
In a general returnless fuel supply system illustrated in FIG. 20, fuel in a fuel tank is delivered to a fuel supply pipe via a fuel pump and a pressure regulator, led into a delivery pipe, and injected from an injector into an internal combustion engine at appropriate injection timing.
On the downstream side of the pressure regulator, the fuel moves only by a distance equivalent to the amount of the fuel injected from the injector and a delay in alcohol concentration change is large. However, on the upstream side of the pressure regulator, because the fuel always circulates through a channel of from the fuel tank to the fuel pump to the pressure regulator to the fuel tank, the delay in alcohol concentration change is extremely small.
Therefore, the delay in alcohol concentration change from the fuel in the fuel tank to the fuel in the injector can be represented by a fuel transfer delay between the fuel supply pipe on the downstream side of the pressure regulator and the fuel supply delivery pipe for injector. A delay in a relatively slim fuel supply pipe is a dead time delay. A delay in a relatively thick fuel supply delivery pipe is a first order delay because the fuel is mixed in the pipe. Such a dead time delay and a first order delay change inversely with a fuel flow rate. When organized by using an integrated injection amount, the delays can be represented by a predetermined function.
FIG. 21 illustrates waveform charts illustrating a delay in alcohol concentration change represented by the predetermined function. The abscissa indicates the integrated injection amount and the ordinate indicates alcohol concentrations in the fuel tank and the injector. When fuel with a high alcohol concentration is supplied to the fuel tank by fueling in a state in which the integrated injection amount is “0”, the alcohol concentration in the fuel tank rises. On the other hand, the alcohol concentration in the injector changes with delays of dead time Q1 and a first order delay Q2. In a period of this concentration change (=Q1+Q2), the integrated injection amount is a predetermined integrated injection amount.
The period of the concentration change in the dead time and the first order delay after the fueling corresponds to several tens minutes at a low fuel flow rate during idling operation of the internal combustion engine and corresponds to several minutes at a high fuel flow rate during high-load operation. Therefore, the period is innegligibly long.
In the conventional apparatuses disclosed in JP 2004-278449 A and U.S. Pat. No. 7,159,623, a period in which the concentration changes with the first order delay after the dead time ends is set based on an integrated injection amount after fueling, and the alcohol concentration is estimated in this set period. While the alcohol concentration estimation is carried out, it is necessary to, for example, inhibit vaporized gas introduction and forcibly carry out air-fuel ratio feedback control, and hence functions of controls other than the concentration estimation degrade. Therefore, the degradation in the functions of the other controls is minimized by discretely carrying out concentration estimation according to the behavior of the concentration change in the first order delay.
In the conventional apparatuses proposed by the inventors, as illustrated in FIG. 21, the integrated injection amount in the period of the dead time fluctuates because of various factors, and hence, when the fluctuation in air-fuel ratio feedback correction coefficient increases after the fueling, it is determined that the dead time has ended and the change in the first order delay has started. A period of the concentration change with the first order delay is set based on the integrated injection amount after the determination of the start. The alcohol concentration is estimated in this set period. As a factor of the fluctuation in the dead time, for example, during the stop of the internal combustion engine, such as during the fueling, the fuel in the fuel supply pipe and the delivery pipe is pushed back to the fuel tank by vaporized fuel caused by transferred heat from the internal combustion engine. The discrete concentration estimation can be carried out according to the behavior of the concentration change in the first order delay irrespectively of the fluctuation in end time of the dead time. Therefore, suppression of the degradation in the functions of the other controls and improvement of concentration estimation accuracy are realized.
A period in which the determination of the start is performed according to the fluctuation in air-fuel ratio feedback correction coefficient is set based on the integrated injection amount after the fueling like a period Q3 illustrated in FIG. 21 to cover a fluctuation width of end time of the dead time. When fluctuation does not occur during the period, the alcohol concentration estimation is not carried out on the assumption that an alcohol concentration change is not caused by the fueling. When it is determined that a concentration change is not caused by the fueling, the degradation in the functions of the other controls due to the concentration estimation is prevented by stopping the concentration estimation.
When the abscissa is organized by the integrated fuel injection amount, as illustrated in FIGS. 22A to 22C, it is understood that an alcohol concentration change in the injector, which occurs when fueling is performed again in a period until the alcohol concentration change after the fueling ends, is superimposition (FIG. 22C) of an alcohol concentration change in the injector in the dead time and the first order delay with respect to an alcohol concentration change in the fuel tank due to the initial fueling (FIG. 22A) and an alcohol concentration change in the injector in the dead time and the first order delay with respect to an alcohol concentration change in the fuel tank due to the refueling (FIG. 22B). The end time of the alcohol concentration change is extended from the end time by the initial fueling. An extended period is a period of an integrated injection amount from the initial fueling to the refueling. As illustrated in FIGS. 23A and 23B, various combinations are conceivable for the concentration changes before and after the refueling and the time of the refueling.
In the conventional apparatuses disclosed in JP 2004-278449 A and U.S. Pat. No. 7,159,623, when fueling is performed again in a period until the alcohol concentration change after the fueling ends, processing of estimating the alcohol concentration in operation is stopped and started from the beginning. Therefore, processing for alcohol concentration estimation corresponding to the alcohol concentration change due to the refueling can be carried out. However, processing for alcohol concentration estimation corresponding to the alcohol concentration change due to the initial fueling is suspended, and the alcohol concentration change due to the initial fueling cannot be sufficiently dealt with. In another case, because the concentration estimation period is set to a period corresponding to the initial fueling while refueling in the period until the alcohol concentration change after the fueling ends is neglected, the initial fueling can be dealt with, but an alcohol concentration change due to the refueling cannot be dealt with.
Further, in the apparatuses proposed by the inventors, the alcohol concentration estimation period is set after the fluctuation in air-fuel ratio feedback correction coefficient after the fueling increases. When the refueling is performed by the time the alcohol concentration change after the fueling ends, the fluctuation in the air-fuel ratio feedback correction coefficient due to the concentration change due to the refueling overlaps the fluctuation due to the concentration change of the initial fueling and cannot be separated. Therefore, an alcohol concentration estimation period corresponding to the refueling cannot be set.
Further, when the period in which the determination of the start is performed according to the fluctuation in the air-fuel ratio feedback correction coefficient is set to the predetermined period after the initial fueling, it is likely that the initial fueling can be dealt with, but the refueling cannot be dealt with. For example, when an alcohol concentration change is not caused by the initial fueling but an alcohol concentration change is caused by the refueling, an implementation period for the determination of the start ends before the fluctuation in air-fuel ratio feedback correction coefficient due to the refueling starts. Therefore, alcohol concentration estimation cannot be carried out. In another case, when the implementation period for the determination of the start is reset to the predetermined period after the refueling when the refueling is performed during the determination of the start after the initial fueling, the refueling can be dealt with, but the initial fueling cannot be dealt with.